This invention relates generally to a guidance system for a projectile launched by firing the projectile from a gun. This invention relates particularly to a sensor system for measuring position and velocity vectors and orientation of a guided gun-launched projectile.
During launch, gun-fired munitions are subjected to extremely high setback accelerations. Here, the term “gun-fired munitions” is also intended to include mortar shells. A gun-fired munition receives all of its kinetic energy during its launch phase. Peak launch accelerations occur during the first 3 to 4 milliseconds from initial movement of the projectile, at which time a projectile has typically moved only a few feet. Shortly after gun barrel exit, the projectile has stopped accelerating and experiences a set-forward acceleration typically about ten percent of the peak set-back acceleration level. Once the round has exited the barrel, no more energy is available to the round during the remainder of the flight, except that for guidance and control actions and when boosters are used to extend the range of the round.
The prior art uses accelerometers and gyroscopes to determine the position and orientation of the round during the flight for guidance and control purposes to ensure precision target acquisition. In the gun-fired munitions applications, these inertia-based accelerometers and gyroscopes are required to withstand extreme harsh launch environments (up to 120,000 g acceleration), yet be sensitive enough to yield the required position and orientation precision up to the target area.
There are two primary challenges with inertial devices currently used as guidance sensors in gun fired munitions for closed loop feedback control. The first challenge of current inertial technologies is gun survivability of devices that have the needed sensitivity for flight measurements. Prior inertial devices are not able to survive when the full-scale dynamic range exceeds 5% of the maximum force experienced during the launch. For guidance applications in gun fired munitions, it is required that the full scale dynamic range during flight be in the order of 0.2% of the maximum force experienced during launch. This challenge is very specific to the environment of a gun-fired munition. The second challenge devices constructed using prior inertial technologies have long settling times that are on the order of a few milliseconds. These limitations of the prior art inertial technologies significantly affect their use as guidance sensors for gun fired munitions.
Precise end game targeting also requires extremely fast activation of the inertia sensor after the initial setback. At an approximate exit velocity of 3000 m/s it is necessary to ensure that the inertia sensors react extremely quickly to avoid badly missing a target. Less than one quarter of a millisecond settling time would significantly advance current inertia sensors for gun fired munition systems—an improvement of more than an order of magnitude that would improve target acquisition by a similar amount.